A desired feature of a programmable digital thermostat is that it be programmable by the, user prior to installing it on the wall and connecting it in the thermostat circuit. Another desired feature is that it contain a backup power source effective to retain the programmed time-temperature schedule in the event that after the thermostat is installed, the normal power source in the thermostat circuit is removed or becomes ineffective.
In some thermostats, both such features are provided by a battery (or several batteries) mounted within the thermostat. A battery also enables maintaining real time. A battery, however, has certain disadvantages. For example, it requires mounting space, sometimes a significant amount, within the thermostat. Thus, the thermostat is sometimes larger than desired in physical size because of the included battery. Also, the battery should be replaced periodically. Such replacement is an inconvenience and an added cost to the user.
In some thermostats, instead of a battery, a large capacitance capacitor, typically, an electric double layer capacitor, is used. The capacitor provides the desired backup power source. The capacitor also enables maintaining real time.
In some thermostats, instead of using a backup power source to retain memory, memory is retained by using an electrically erasable programmable read-only memory (EEPROM) chip. An EEPROM chip is a memory device, not a power source. While the chip requires no power to maintain memory, it does require power to effect a change to the memory. Typically, such thermostats also include a small capacitance capacitor for maintaining real time.
A particular disadvantage of those thermostats not utilizing a battery is that they cannot be programmed by the user prior to installation. Specifically, whereas a battery has an inherent output voltage to provide power to the thermostat so as to enable such prior programming, a capacitor has no output voltage until it is charged. Also, as noted above, an EEPROM chip requires power to effect a change to its memory.